Nearly all practical transceivers require some form of filtering. Up to the date, a majority of their radio-frequency (RF) and intermediate-frequency (IF) bandpass (BP) filters are realized as off-chip ceramic or surface-acoustic wave (SAW) devices. These devices do not have a practical way to change their center frequency (CF) and/or their bandwidth (BW).
In cases when a BP filter is required to change its CF and/or its BW discrete component solutions are used that apply varactors as tunable capacitors that together with external coils (inductors) allow to tune the filter CF. Since the off-chip filter components can easily pick up noise and radio interference they need to be shielded by a metal can. This requirement unavoidably increases the cost and dimensions of the transceiver module.
Widely tunable LC BP filters for applications operating at few tens of MHz are not easy to integrate on-chip because of the prohibitively large physical size of the required on-chip inductors. Moreover, their tuning range is limited by that of the varactors that varies form a few to few tens of percent, which may not be adequate for applications such as TV tuners, where hundreds of percent of CF change is required. Additionally, due to limited Q-factors of on-chip inductors and varactors the active Q-enhancement schemes must be added that further limit the filter noise and distortion performance and the resulting dynamic range (DR). Such schemes also increase the filter power consumption. Finally, the viable at high-frequency BP filter synthesis methods such as mutually coupled resonators suffer from the asymmetry of the transfer function due to frequency dependent loss of resonators as well as to parasitic C couplings. Moreover, for these class of circuits the Q-tuning often fails due to instability or lack of convergence unless special techniques are applied.
It would be much more convenient if a BP filter was realized in a pure active technique such as in gm-C, active R-C, MOSFET-C, gm-active-C or other, to improve its tunability range and to obtain a flat symmetrical transfer function. It is not uncommon to achieve a tuning range in excess of 1000 percent when using one of these techniques. This class of circuits is also free form Q-tuning problems typical for Q-enhanced LC mutually coupled resonators. However, the noise, distortion, the resulting DR and power consumption are all worse than that of LC Q-enhanced BP filters.
Another obstacle in the integration of RF filters is the need to adjust their CF and their Q-factors. The accuracy problems of the required tuning systems may result in the whole filter not meeting the stringent system specifications over process, voltage supply and temperature variation (PVT). Due to the matching errors between the Master and Slave the most frequently used indirect tuning, or Master-Salve (M-S) schemes suffer from significant accuracy errors up to 5% for CF and up to several tens of percent for Q-factor. Naturally, for majority radio applications such a modest accuracy is not acceptable. Additionally, the reference feed-through degrades the overall noise performance of the filter. The typically achievable S/N ratio for an active filter tuned with a M-S scheme is about 40 dB.
In the case of presented BP filters instead of using the M-S scheme the tuning accuracy can be substantially improved by directly tuning the filter as a self-tuned filter or as a self-tuned with common-mode (CM) signals filter as shown in U.S. Pat. No. 5,608,665 by passing the CM reference through the filter while simultaneously processing the differential signal. The simplest configuration is obtained by two single-ended structures forming a pseudo-differential filter. Because these tuning schemes are free from the M-S matching errors the expected accuracy of self-tuned frequency- and Q-tuning systems could be as good as 0.5% and 2% respectively.